Frequency modulation receiver



Patented June 27, 1939 UNITED STATES FREQUENCY MODULATION RECEIVERMurray G. Crosby, Riverhead, N. Y., assigner to Radio Corporation ofAmerica, a corporation of Delaware Application October 30, 1937, SerialNo. 171,820

14 Claims.

This disclosure concerns a frequency modulation receiver of the typewherein a phase modulation receiver is employed and a correction isapplied to convert the phase modulation output to a frequency modulationoutput. However, instead of applying the correction at the audiofrequency output or at the intermediate frequency as has been done inthe past, the correction is applied as a frequency modulation of aheterodyne osio cillator at such a phase as to oppose or cancel themodulation on the incoming wave to the degree required to convert theoutput from a phase mod` ulation output to a frequency modulationoutput.

In the prior art, frequency modulation has been received on a slopingfilter receiver in which the frequency deviations are converted toamplitude deviations by the sloping filter. Conventional amplitudemodulation detection meth- "Q0 ods are then employed to detect theresulting amplitude modulation. Crosby United States Patent #2,060,611dated November 10, 1936; application #114,894 led December 9, 1936,Patent #2,154,398, patented April ll, 1939; and Crosby application#114,778 led May 26, 1937, Patent #2,138,341, patented November 29, 1938describe this type of receiver.

Frequency modulation is also received on retard circuit receiverswherein the modulated wave is divided into two parts one of which ispassed through a retard circuit and recombined with the unretarded wave.The resulting amplitude modulation is then detected by the ordinarymethods. Crosby application #618,154 led June 20, 1932; Patent#2,065,565 dated December 29, 1936; and Patent #2,087,429 dated July 20,1937, describe such receivers.

A third method of receiving frequency modulation is by means of acorrected phase modulation receiver. Due to the inherent similaritiesbetween frequency and phase modulation, frequency modulation may bereceived on a phase modulation receiver, but the output will bedistorted with respect to they amplitudes of the vari- 45'ous modulationfrequencies. The audiooutput will be inversely proportional to themodulation frequency so that the lower modulation frequencies willappear overmodulated and the higher frequencies undermodulated. Tocorrect 50 this distortion, an audio correct circuit may be inserted inthe receiver output terminals which attenuates the output frequenciesinversely proportional to the frequency. In other words the Acorrectioncircuit would diminish' th-e output of lthe 'lower modulationfrequencies and increase (Cl. Z50-20) that of the higher frequencies.VIn this way the distortion caused due to the fact that the fre quencymodulation was being received on a phase modulation receiver iscompensated for.

The receiver of this disclosure employs a phase modulation receiver toreceive frequency modulated waves, but instead of applying an audiocorrection, a form of correction is applied which effectivelyunmodulates the intermediate frequency of the receiver by the requisiteamount to make the receiver output faithful to the true modulationapplied at the frequency modulation transmitter. In order to accomplishthis effect, part of the output of the phase modulation detector isutilized to frequency modulate an oscillator of a superheterodynereceiver. This modulation is applied to the said oscillator at such aphase as to modulate the resulting intermediate frequency of thereceiver in such a direction that the modulation on the intermediatefrequency is decreased. In general, the direction of modulation of theoscillator is the same as the modulation already present on th-e wavedue to the frequency modulation at the transmitter. The phase modulationdetector has an output when receiving frequency modulated waves whereinthe low modulation frequenciesare over accentuated (amplitudes inverselyproportional to frequency). When these potentials having thesecharacteristics are used to modulate the oscillations used for beatingpurposes, the lower frequency potentials have greater effect, being ofrelatively greater amplitude than the higher modulation frequencies.This', in effect, straightens out the demodulation characteristic of thereceiver which without my novel method and means is adapted to phasemodulation reception and with my method and means to frequencymodulation reception. Consequently, the said frequency modulation of theintermediate frequency is effectively unmodulated into phase modulationso that the receiver is receptive to frequency modulation.

In describing my method and means for receiving frequency modulatedwaves, reference will be made to the attached drawing wherein bothfigures each show receivers including the Aelements essential tooperation in accordance with the principle outlined above.

The circuit of Figure 1 shows a specific embodiment wherein a phasemodulation receiver of the synchronized local oscillator type isutilized.

Units l, 2, 3, and 4 make up a conventional supery heterodyne type ofreceiver. The intermediate frequency energy from amplifier 4 is fed to asecond detector 6 connected with a second local oscillator 8 and fromthe output of the second detector 6 to differential detectors 9 and I0Via transformer 'I, the primary P of which is coupled to 6 andthesecondary S of which is coupled to the grids I 4 and I 6 of tubes 9and I6. Intermediate frequency energy from oscillator I2 is also fed tothe grids I4 and I6 of these differential detectors via common legtransformer I8. In operation, the frequency of 3 and 8 and I2 are suchthat the mean frequency at the output of 6 is equal to the meanfrequency of I2. In other words the output of 6 is synchronized with theoscillations produced by I2. The differential detector output energysupplied from the anodes I3 and 28 which appears across resistors 2loperates through circuit 24 to frequency modulate the oscillator 8. Thisdifferential detector energy varies in accordance with the phasedeviations between the carrier oscillator I2 and the output of seconddetector 6. The operation of this type of differential detector is morecompletely described in Crosby Patent #2,065,565 dated December 29,1936. In brief, these detectors produce a detected voltage which has onepolarity when the phase between the frequency modulated intermediatefrequency energy and the oscillations from the carrier oscillator I2deviates in one direction, and a voltage of opposite polarity when thisphase deviates in the other direction. Consequently, by applying thisdetected voltage to the grid of a modulator tube, say in 26, whichmodulates the frequency of the high oscillator 8, a frequency correctionmay be applied to the said oscillator which will bring the intermediatefrequency at the output of 6 back into synchronism with the oscillationsfrom oscillator I2. The oscillator 8 and the modulator 26 may be of anytype. For example, modulators of the reactance tube type such as shownin my United States applications #165,056 led September 22, 1937 and#167,344 filed October 5, 1937, Patent #2,156,374, patented May 2, 1939and Patent #2,156,375, patented May 2, 1939, respectively, may be usedwith any oscillator such as for example an oscillator as shown in saidapplication.

n ordinary automatic tuning or frequency control practice, a timeconstant circuit is inserted in the leads 24 to the modulator 26 of thehigh oscillator 8. This is to prevent the signal variations due to theapplied modulation from varying the frequency of the high frequencyoscillator and to allow only the slow and more gradual drifts fromsynchronism to operate the control. However, in the present receiverthis time constant circuit is eliminated and the signal modulations inthe output resistors 2l of detectors 9 and I8 is allowed to act throughthe modulator 26 to modulate the high frequency oscillator. This step isthat one which enables this type of receiver to receive frequencymodulation. The frequency modulation which normally comes in at theintermediate frequency is automatically corrected to phase modulationina manner similar to that in which an automatic volume control with afast time constant removes amplitude modulation. The normal output ofthe phase modulation detectors would be very strong in low modulationfrequencies and weak in the highs due to the fact that frequencymodulation was being received on a phase modulation detector. When thisoutput is fed back to the high frequency oscillator to unmodulate it,the depth of modulation for the lower modulation frequencies of theintermediate frequency modulated wave is reduced to a value which makesthe modulation phase modulation.

The potentials appearing in resistors 2| are4 supplied by way of circuit24 to a controlling grid 3| and cathode 42 of a tube 36 in themodulator. The tube 39 is a reactance tube and controls in a mannerwhich will now be described the frequency of the oscillations producedby the oscillator 8. The oscillator 8 is of the grounded grid type andincludes a tube 32 having its grid 34 and anode 36 coupled in afrequency stabilizing and regenerative circuit 38 as shown. A point onthe circuit 38 is coupled to the detector in 6, this circuit beingcompleted by way of ground. The grid end of circuit 33 is also groundedfor radio frequency by connecting one end of the grid leak and condenserarrangement 48 to ground. The reactance tube 39 has its anode 44 coupledto its grid i6 by phase shifting condenser 48 and resistance 99. Thegrid 46 is connected to the cathode 42 by way of bias resistor 37 andcondenser 43. The anode 94 is coupled to the high potential end of 38while the cathode 42 is grounded by by-pass condenser C. The value ofresistance 49 is high as compared to the reactance of condenser 43 forthe frequency produced in 38 so that the current through this circuit islargely resistive and is in phase with the voltage. However, the voltagedrop through 43 leads the current by substantially 90 and a phasequadrature relation between the radio frequency potentials on anode 94and grid 46 necessary for the reactive effect is obtained. The tubereactance, that is, the reactance tube 39, shunts the circuit 38 andconsequently the reactance tube controls to some extent, the frequencyof oscillations produced by 32 and 3S. This reactive effect which may beconsidered inductive or capacitive is in turn controlled by thepotentials supplied from 24 to the grid 3l of tube 39. The potentialsupplied to 3| Varies in accordance with the potentials in 2I which inturn are characteristic of frequency modulations on the wave energy asconverted by a phase modulated wave demodulator. These potentials,accordingly, act through reactance tube 30 to unmodulate theoscillations produced in tube 32 and circuit 38. Since the plate 44 isconnected to the plate 36 andv the grid 46 is eX- cited by voltagesdisplaced in phase relative to the voltages in circuit 38 the platecurrent in tube 39 is likewise out of phase with the voltage in 38 andthe control tube 30 looks like a reactance to the circuit 38.

Similar arrangements have been shown in my United States application#124,967 filed February l0, 1937, Patent #2,065,565 dated December 29,1936, Patent #2,085,908 dated June 29, 1937, application #136,578 ledAugust 13, 1937, application #209,919 filed May 25, 1938, application#167,344 led October 5, 1937, Patent #2,156,375, patented May 2, 1939.

The above unmodulating circuit converts the intermediate frequency ofthe'receiver from frequency modulation to phase modulation. Hence,

the phase modulation detectors 55 and 56 willI receive the modulationwithout distortion. Oscillations from I2 are supplied by phase shifter52 to 54 in phase on the grids 60 and 62. Signal modulated waves aresupplied from the output of 6 in phase opposition to the grids 68 and 62from 53. The anodes of 55 and 56 are connected to primaries 64 and 66 oftransformer 57. Phase adjuster 52 enables the reception of eitheramplitude or frequency modulation. Since the phase detectors 9 and I0would be insensitive to applied amplitude modulation, there would be nounmodulating effect due to amplitude modulation and the detectors 55 and56 would only function as automatic frequency control detectors. Thencarrier exalted amplitude modulation reception could be effected byeither adjusting in 52 the carrier from I2 at 90 with the intermediatefrequency energy from 6 and throwing switch 59 to the parallel position,or by adjusting the carrier to in one detector and 180 in the other withswitch 59 in the push-pull connection. The output of detectors 55 and50,Y combined in transformer secondary 51, is made available forutilization in phones or other utilization device 58.

The receiver of Figure 2 employs a phase modulation receiver of thecarrier filter type. By this is meant that the synchronized oscillatorI2 of Figure 1 has been dispensed with and the modulated wave carrierisolated and used in place of the locally produced synchronousoscillations. Units I, 2, 3, 4, comprise a heterodyne receiver includinga bandpass intermediate frequency amplifier 4 which is tuned to pass thefrequency modulated wave energy of lower Ifrequency. The oscillator tube60 and circuit 65 comprising a variable condenser and inductanceconnected as shown, the oscillator 60 and line 69 with the detector 6,constitute a superheterodyne receiver. Tube 68 is the frequencymodulator tube for the oscillator 60. This tube varies the eifectivecapacity produced by condenser 61 when a variationof the voltage of grid10 varies the plate resistance of the tube 68. The output of the seconddetector 6 is fed via transformer 16 to the grid 'I8 of coupling tube 80and from the anode 19 of 80 to the input electrodes 82 of a fourelectrode crystal lter. 8| is the crystal. Shield 84 shields the crystalinput electrodes 82 from the crystal output electrodes 83 so thatcapacitive coupling is eliminated. The output electrodes 83 of thecrystal filter feed energy from which all modul'ations have been removedor stripped to the anodes |00 and I0| of the dupleX-diode-pentode tube96 in a push-pull manner much the same as transformer 1 of Figure 1operates to feed unfiltered energy to the grids i4 and I6 of theAtriodes 9 and I0, instead of diode detectors as in Figure 2. Theunfiltered energy is fed to the diode differential detector by feedingit tothe grid Yof the pentode part of tube 96 so that it appears oncathode choke 81. By feeding the unfiltered energy in the cathodecircuit in this manner, the same result as that of the common legtransformer I8 of Figure 1, is accomplished. The diode detector energyis taken from resistors 85 connected in shunt to |00 and |02 andgrounded at |06 and fed via line 24 to the grid 10 of tube 68 to controlits output reactance which shunts the frequency determining circuit 65`of 60 and unmodulates the oscillator 60. Since theregis an inherentphase shift of 90 in this type of crystal filter, there is no need for aphase shifter in this circuit because the phase quadrature relationbetween the energy direct from 16 and the energy from 16 passed by thecrystal is inherently obtained in the crystal. The detected audio outputis taken from resistor 85 and fedrvia line 92 to the grid 9| of couplingtube 88 where itis made available for utilization at jack 89.

While it will be obvious that an unlimited selection of frequenciesmaybe used in the various oscillators and for the received modulated'wave and that I contemplate the use of all appropriate frequencies, Ihave found that it is satisfactory to use oscillators at 3 of such afrequency that the output of 2 and 4 is of about 460 kc. may thenoperate at say, 560 kc. with an output from 6 of 100 kc.

It is understood that the above described embodiments omit all detailswhich would be common practice in the art of frequency and phasemodulation reception. For instance, an amplitude limiter could beinserted in both circuits following the intermediate frequency amplier.Also any type of phase modulation receiver,y such as those of CrosbysUnited States Patent #2,114,335 dated April 19, 1938; Patent #2,064,-106 dated December 15, 1936; Patent #2,112,881 dated April 5, 1938;application #124,967 filed February 10, 1937; and Patent #2,076,175dated April 6, 1937, could be'made to operate as frequency modulationreceivers in the manner described in this disclosure.

By applying an audio or intermediate frequency correction, such as aredescribed in Crosby United States application #618,154 led June 20,1982, and application #124,967 filed February 10, 1937 respectively, thereceivers of this disclosure may be corrected so that they will receivephase modulation. For instance, an audio circuit, which amplies theoutput frequencies inversely propor-` tional to frequency, could beinserted in the output leads of either receivers of this disclosure, orany other employing the ideas herein disclosed, so as to make thereceiver receive phase modulation. This appears to be a ratherround-about way of receiving phase modulation-.that is, unmodulating aphase modulation receiver to make it receive frequency modulation andthen applying a correction circuit to make it receive'phase modulationagain. However, there are advantages in this process. One advantage isin the fact that automatic frequency control is enhanced. For instancein the case ofthe synchronized oscillator receiver of Figure 1, theaudio frequency control circuit must be very faithful or anoif-synchronismcondition will cause an annoying audio heterodyne in thereceiver output. By allowing the audio frequency control circuit to befast in the manner of this unmodulatedcorrected receiver, the audiofrequency control has a better control capability.

The principle of operation here is equally applicable to systems formodulating an oscillator of a frequency modulated wave demodulator whenthe latter is to be used for phase modulated waves to unmodulate thesaid waves in such a manner that the output of the frequency modulatedwave demcdulator will be a correct representation of the signals used atthe transmitter to modulate the wave'. Normally when phase modulatedwaves are received on a frequency modulated wave receiver the resultingpotentials vary in amplitude directly as the frequency of the modulatingpotentials vary. Thus, potentials are then fed back to the oscillator tomodulate the same in a sense to cause the output potentials of thereceiver to be truly representative of the modulating potentials used atthe transmitter. In the claims the term length modulation has been usedto co-ver both systems.

I claim:

1. In a wavelength modulated wave demodulating system in combination, adetector having electrodes including an output electrode, an alternatingcurrent circuit connected with certain The modulated oscillators 28 and60 of said electrodes, an impedance connected with certain of saidelectrodes, means for impressing Wavelength modulated Wave energy onsaid alternating current circuit, the characteristic of said detectorbeing such that the potentials on said output electrode vary inamplitude nonlinearly as the frequency of the modulations on thewaveenergy impressed on said alternating current circuit varies, and meansfor unmodulating said wave energy in accordance with said potentials.

2. In a frequency modulated Wave demodulating system in combination, adetector having electrodes including an output electrode, an alternatingcurrent circuit connecting certain ones of said electrodes in an inputcircuit, an impedance connected with electrodes of said detector, meansfor impressing frequency modulated Wave energy on said input circuit,the characteristic of said detector being such that the potentials onsaid output electrode vary in amplitude inversely as the frequency ofthe modulations on the Wave energy impressed on said alternating currentcircuit varies, and means for unmodulating said Wave energy inaccordance with said potentials.

3. In a frequency modulated Wave demodulating system in combination, adetector having input and output electrodes and having an alternatingcurrent circuit connected to its input electrodes and an impedanceconnected to its output electrodes, means for impressing frequencymodulated Wave energy on said alternating current circuit, thecharacteristic of said detector being such that the potentials acrossthe impedance in the output thereof vary in amplitude inversely as thefrequency of the modulations on the Wave energy impresed on saidalternating current circuit varies, and means for unmodulating said Waveenergy in accordance with said potentials.

4. Means for adapting a phase modulated wave receiver of the heterodynetype including a local oscillator coupled with a detector and a phasemodulated Wave demodulator coupled to the detectcr and responsive to thedifference frequency, to the reception of frequency modulationcomprising, means for impressing frequency modulated Wave energy on saiddetector and frequency modulated Wave energy of the difference freuquency on said demodulator, and means for modulating the frequency ofthe local oscillator in acn cordance with potentials from the output ofsaid phase modulated Wave demodulator.

5. Means for adapting a phase modulated wave receiver of the heterodynetype including a local oscillator coupled With a detector responsive tofrequency modulated Wave energy and a phase modulated wave demodulatorhaving an input coupled to the detector and responsive to the differencefrequency and an output including an impedance, to the reception of thefrequency modulated Wave energy comprising, means for impressingfrequency modulated Wave energy on said detector, and means couplingsaid impedance to said oscillator to modulate the frequency of the samein accordance With the potentials produced in the output of said phasemodulated Wave demodulator and in a direction similar tothe direction offrequency modulations on said waves.

6. Means for adapting a phase modulated Wave receiver of the heterodynetype including a local oscillator coupled With a detector excited byfrequency modulated Wave energy and a phase mod ulated Wave demodulatorcoupled to the said detector and responsive to the difference frequency,to undistorted reception of frequency modulated Wave energy comprisingan impedance connected with the output of said phase modulated Wavedemodulator, and a control circuit connecting said impedance to saidlocal oscillator to control the frequency thereof in accordance with thepotentials in the output of said demodulator and in a direction tounmodulate the frequency modulations on said wave energy.

7. Means for adapting a phase modulated receiver of the heterodyne typeincluding a local oscillator coupled with a detector responsive tofrequency modulated Wave energy and a phase modulated Wave demodulatorhaving an input coupled to the detector and responsive to the differencefrequency and an output including an impedance to the reception of thefrequency modulated wave energy comprising, means including a reactancetube having control electrodes con-l nected to said impedance andelectrodes coupled to said oscillator to modulate the frequency of saidoscillator in accordance With the potentials produced in the output ofsaid phase modulated Wave demodulator and in a direction similar to thedirection of frequency modulations on said Waves.

8. In a system for demodulating frequency modulated Wave energy, incombination, a rst detector with means for impressing frequency 1modulated Wave energy thereon, a local oscillator coupled with saiddetector for producing With said frequency modulated Wave energy a beatfrequency, a second detector having an alternating current input circuitcoupled to the output of said first detector and having an outputcircuit inoluding an impedance, said second detector being excited byWave energy of beat frequency, the character of said second detectorbeing such that the potentials in the output circuit thereof vary inamplitude inversely as the frequency of the modulations on the beatfrequency impressed on the input circuit thereof, and means coupling theoutput of said second detector to said local oscillator to modulate thefrequency of the same in a sense to unmodulate the frequency modulationson said Wave energy of beat frequency.

9. In a system for demodulating frequency modulated Wave energy incombination, a first,V

detector with means for impressing frequency modulated Wave energythereon, a local oscillator coupled with said detector for producingwith said frequency modulated wave energy a beat frequency, a seconddetector having an alternating current input circuit coupled to theoutput of said rst detector and to a source of oscillations of afrequency substantially equal to the mean frequency of said beatfrequency, said second detector having an output circuit including animpedance, said second detector being excited by frequency modulatedWave energy of beat frequency and Wave energy of the mean beatfrequency, the character of said second detector being such that thepotentials in the output circuit thereof vary in amplitude inversely asthe frequency of the modulations on the beat frequency impressed on theinput circuit thereof, and means coupling the output of said seconddetector to said local oscillator to modulate the frequency of the samein a sense to unmodulate the frequency modulations on said Wave energyof beat frequency.

10. A system as recited in c1aim'9 wherein said source of oscillationscomprises an oscillator substantially synchronized with said mean beatfrequency.

11. A system as recited in claim 9 wherei said source of oscillationscomprises a crystal lter having an input fed by energy from the out putof said first detector and an output coupled to said second detector.

12. In a system for demodulating frequency modulated Wave energy'incombination, a rst detector with means for impressing frequencymodulated Wave energy thereon, said rst Ydetector having an output, alocal oscillator coupled with said first detector for producing withsaid frequency modulated Wave energy a beat frequency, a second detectorhaving an alternating current input circuit coupled to the output ofsaid first detector, and a second alternating circuit input including ahighly selective filter coupled to the output of said rst detector, saidsecond detector having an output circuit including an impedance, saidsecond detector being excited by wave energy of beat frequency and Waveenergy of said beat frequency as passed by said filter, the character ofsaid second detector being such that the potentials in the outputthereof vary in ampltiude inversely as the frequency of the modulationson the beat frequency impressed on the input thereof Varies, and meanscoupling the output of said.v second detector to said local oscillatorto modulate the frequency of the same in a sense to unmodulate thefrequency modulations on said Wave energy of beat frequency.

13. A system as recited in claim 12 wherein said filter shifts the phaseof the energy passed thereby substantially 14. The method ofdemodulating alternating current, the frequency of which has beenmodulated in accordance with signals, including the steps of, beatingsaid frequency modulated alternating current With oscillations, of amean frequency substantially equal to the mean frequency of the saidfrequency modulated alternating current, in -phase displaced relation,deriving from said beating action voltages which vary in amplitudeinversely as the frequency of the signals used to frequency modulatesaid alternating current vary, and modulating the frequency ofsaidoscillations used in said beating action in a sense to tend to make thederived voltages vary linearly in accordance With the frequency `of thesignals used for modulating the frequency of said alternating current.

MURRAY G. CROSBY.

